1. Field of the Invention
The present invention relates to a spacer grid used for placing and supporting a plurality of nuclear fuel rods within a nuclear fuel assembly, and more particularly, to an anti-fretting wear spacer grid with canoe-shaped springs capable of preventing the fretting wear of fuel rods, thereby avoiding the damage of the fuel rods.
2. Background of the Related Art
A nuclear reactor is a device that artificially controls the chain reaction of the nuclear fission of fissile materials, thereby achieving a variety of use purposes such as the generation of heat, the production of radioisotopes and plutonium, the formation of radiation fields, or the like.
Generally, enriched uranium that is obtained by raising a ratio of uranium-235 to a range between 2% and 5% is used in a light water nuclear reactor. The uranium is molded to a cylindrical pellet having a weight of 5 g and is processed to a nuclear fuel used in the nuclear reactor. Numerous pellets are piled up to form hundreds of pellet bundles and then put into a cladding tube made of Zircaloy being at a vacuum state. After that, a spring and a helium gas are put thereinto, and an upper end plug is welded thereon, thereby making a fuel rod. The fuel rod is finally surrounded by a nuclear fuel assembly and then burnt up within the nuclear reactor through nuclear reaction.
The nuclear fuel assembly and the parts therein are shown in FIGS. 1 and 2. FIG. 1 is a schematic view showing a general nuclear fuel assembly, FIG. 2 is a top plane view showing the spacer grid, and FIG. 3 is a cut-off perspective view showing the spacer grid.
Referring to FIG. 1, the nuclear fuel assembly basically includes a frame body comprised of a top nozzle 4, a bottom nozzle 5, guide thimbles 3, and a plurality of spacer grids 2, and a plurality of fuel rods 1 held longitudinally in an organized array by the spacer grids 2 spaced along the length thereof in such a manner as to be supported by means of springs 6 (see FIGS. 2 and 3) and dimples 7 (see FIGS. 2 and 3) disposed within the spacer grids 2. So as to prevent the formation of the scratches on the fuel rods 1 and the generation of the damage on the springs within the spacer grids 2 upon assembling the nuclear fuel assembly, thereafter, the fuel rods 1 have a locker applied thereon and are then inserted longitudinally into the frame body of the nuclear fuel assembly. Next, the top and bottom nozzles are secured to the opposite ends of the nuclear fuel assembly, thereby finishing the assembling procedure of the nuclear fuel assembly. Then, after the locker of the finished assembly is removed, the distances between the fuel rods 1, the distances between the fuel rods 1, the distortion of the nuclear fuel assembly, the total length thereof, and the dimension thereof are checked out, thereby finishing the manufacturing procedure of the nuclear fuel assembly.
Referring to FIGS. 2 and 3, the spacer grid is made in a lattice by coupling a plurality of slots (not shown) formed by a plurality of strips (thin metal plate) connected with one another so as to define a plurality of space portions into which the fuel rods 1 are held longitudinally thereby. About 10 to 13 spacer grids are arranged along the length direction of the nuclear fuel assembly and welded to the guide thimbles 3 having a length of 4 m. The springs 6 and the dimples 7 are regularly formed on each space portion defined by the spacer grid 2, such that as they are brought into contact with the fuel rod 1 (see FIG. 1), the distance between the fuel rods 1 is maintained and arranged at their defined position. Further, the fuel rods 1 are fixed by the elasticity of the springs 6.
On the other hand, the recent development of the nuclear fuel is aimed to obtain high burn-up performance and integrity. So as to develop high burn-up fuel, there are a variety of methods of enhancing the thermal transmission efficiencies from the fuel rods to coolant. Many of the methods are introduced wherein the improvement in the flow characteristics of coolant around the fuel rods is effectively accomplished by attachment of mixing blades to the spacer grid, a change of the shape of the mixing blades, or an appropriate configuration of coolant flow channels in the spacer grid.
However, the above-mentioned methods of enhancing the thermal transmission efficiencies of nuclear fuel assemblies also generate turbulences in coolant flowing around the fuel rods, and the turbulences of the coolant undesirably cause flow-induced vibration by which the fuel rods are vibrated.
The flow-induced vibration of the fuel rods 1 causes the fuel rods to slide against their contact surfaces at which the fuel rods are brought into contact with the springs and dimples of the spacer grids, such that the contact surfaces are partially abraded to cause fretting wear of fuel rods to occur, which results in the damage on the fuel rods. That is, the above-mentioned methods for improving the thermal performance so as to develop the high burn-up fuel result in the acceleration of the damage of the fuel rods.
As the burn-up is conducted, on the other hand, the irradiation growth of the spacer grid is performed transversely. Also, the fuel rods are repeatedly contracted by the radial creeps in the burn-up process in the reactor, that is, by the high pressure caused by the coolant in the reactor, and extended radially by the expansion of fuel pellets, such that the outer diameters of the fuel rods have irregular directionality, which generates the gap between the springs/dimples of the spacer grid and the fuel rod, thereby causing much fretting wear.
So as to reduce the fretting wear, the contact length between the fuel rod and the springs/dimples is extended longitudinally, and otherwise, the surface-contact therebetween is generated, so that even though the fretting wear occurs, the wear depth is substantially reduced even under the same wear area.
FIGS. 4a and 4b show the conventional spacer grid having a spring shape generally used, wherein the linear contact length between the fuel rod and the springs/dimples in the spacer grid is extended longitudinally. The springs protrude horizontally and vertically on a grid surface of the strip and support a nuclear fuel rod.
In the conventional spacer grid, each spring is flat-shaped in a manner of linear contact with the fuel rod, so that the linear contact manner more effectively protects the fuel rod from fretting wear. However, actually, the spring is irregular on the flat surface thereof to obtain an elastic force when supports the fuel rod, as shown in FIG. 4a. Therefore, the initial contact between the fuel rod and the spacer grid are not linear contact, but are three-point contact. As the fretting wear is developed, the three-point contact is changed into the linear contact. At this time, the fretting wear of the fuel rod is accelerated due to the initial three-point contact.
On the other hand, there has been proposed a method for reducing the fretting wear by the generation of the surface contact of the spacer grid with the fuel rod, which is disclosed U.S. Pat. No. 6,606,369 (hereinafter, referred to as ‘prior art’) entitled ‘Nuclear reactor with improved grid, as filed Mar. 6, 2002.
According to the prior art, as shown in FIGS. 5a and 5b, a spring 62 has a curved surface in such a manner as to have the surface contact with the fuel rod 1, which more effectively prevents the movements of the fuel rod in axial and transverse directions caused by the flow-induced vibration, in comparison with the conventional point or linear contact manner.
According to the prior art, however, the contact surface of the spring 62 and the dimples 72 with the fuel rod 1 is formed in the same length as the curvature radius of the fuel rod 1, which really makes it impossible to form or maintain the surface contact in an accurate manner by the manufacturing tolerance of the spacer grid and by the variation of the roundness of the fuel rod 1 and also to maintain constant roundness and curvature radius during the burn-up process of the fuel rod.
Further, in case where the contact portion of the spring does not have a theoretically complete curved surface, the irregular linear or point contact of the spring 62 with the fuel rod 1 is formed to cause unexpected fretting wear to happen.